[Lens] Math operation in formula nulls if any of the operations has null results

[dej611]

Describe the bug:

When using Lens formula with a math operation, if any of the operators has a null result, that leads to a null the entire formula.
For instance, the following formula will be given a lower result (null) compared to a subset of it without the math operation:

average(bytes, kql='geo.dest: "US"') + average(bytes, kql='geo.dest: "DE"')

Here you can compare with the average(bytes, kql='geo.dest: "US"') counter part (column 3 of the table):

Expected behavior:

I can consider two expected behaviour here:

• not null the full formula, rather consider it as 0 in case of null for any operator
• add some documentation about the behaviour, either in-app or in the Lens advanced page
• add a new function to fallback null values to a given value (default( 0, average(bytes, kql='geo.dest: "US"')) + ...)

[elasticmachine]

Pinging @elastic/kibana-vis-editors @elastic/kibana-vis-editors-external (Team:VisEditors)